The reduction of the melting point in nanometer-sized metal and molecular clusters has been the subject of numerous theoretical studies and simulations (Ph. Buffat and J.-P. Borel, Phys. Rev. A (1976) 13:2287; M. Wautelet, J. Phys. D (1991) 24: 343; F. Ercolessi, W. Andreoni and E. Tossati, Phys. Rev. Let. (1991) 66:911; R. S. Berry, J. Jellinek and G. Natanson, Phys. Rev. A (1984) 30: 919). Work in the semiconductor area has focussed on II-VI and III-V materials, such as CdS (M.L. Stiegerwald et al, J. Am. Chem. Soc. (1988)110:3046; V. L. Colvin, A. N. Goldstein and A. P. Alivisatos, J. Am. Chem. Soc. (1992)114:5221) and GaAs (M. A. Olshavsky, A. N. Goldstein and A. P. Alivisatos, J. Am. Chem. Soc. (1990) 112: 9438). The reduced melting temperature in these binary semiconductor systems has been demonstrated (A. N. Goldstein, C. M. Echer and A. P. Alivisatos, Science (1992)256:1425), albeit with a material dependent tendency towards disproportional prior to melting, such as in the case of GaAs (A. N. Goldstein, Ph. D. dissertation, University of California at Berkeley (1993)).
For several reasons, however, these studies have yet to be extended to more open systems, including the group IV semiconductor materials, in which bonding is predominantly covalent. First, existing models rely on the concept of surface tension, a term difficult to define for covalent nanocrystals. A large proportion of surface atoms has an effect on the thermodynamic properties and, therefore, plays a significant role in the high temperature behavior of such systems.
Secondly, to carry out such investigations, techniques must be developed to synthesize narrow size distributions of the appropriate nanocrystal precursors, and these techniques have not been perfected for the group IV semiconductors. Si and Ge nanocrystals have been produced through evaporative processes (K. A. Littau et al, J. Phys. Chem. (1993) 97:1224; M. Fujii et al, Jap. J. Appl. Phys. (1991)30:687; R. A. Zhitnikov, USSR patent 157,336), but due to the covalent nature of these materials, significantly reduced melting temperatures have not been demonstrated.
The group IV materials carbon, silicon and germanium all exhibit covalent bonding, but the successive addition of filled orbitals transforms the isostructural compounds from insulator through semiconductor to a more metallic character. The effect of these changes in electronic structure on the nanocrystal thermodynamic property of melting is not obvious. Moreover, Group IV materials cannot exhibit disproportion. Disproportionation is the selective volatilization of one type of atom in heteratomic material, resulting in an enrichment in the more stable species. Instances in which selective loss of small amounts of one type of atom from the nanocrystal surface entropically drives the melting process cannot exist for the Group IV materials.